Not applicable.
The present invention relates generally to methods and apparatus for controlling borehole pressure in wells. More specifically, the present invention relates to methods and apparatus employing continuous real-time pressure while drilling measurements to bring borehole pressure back into control after borehole pressure is below pore pressure or greater than fracture pressure.
A drilling fluid is typically used when drilling a well. This fluid has multiple functions, one of which is to provide pressure in the open wellbore in order to prevent the influx of fluid from the formation. Thus, the pressure in the open wellbore is typically maintained at a higher pressure than the fluid pressure in the formation pore space (pore pressure). The influx of formation fluids into the wellbore is called a kick. Because the formation fluid entering the wellbore ordinarily has a lower density than the drilling fluid, a kick will potentially reduce the hydrostatic pressure within the well and allow an accelerating influx of formation fluid. If not properly controlled, this influx is known as a blowout and may result in the loss of the well, the drilling rig, and possibly the lives of those operating the rig. Therefore, when formation fluid influx is not desired (almost always the case), the formation pore pressure defines a lower limit for allowable wellbore pressure in the open wellbore, i.e. uncased borehole.
The open wellbore extends below the lowermost casing string, which is cemented to the formation at, and for some distance above, a casing shoe. In an open wellbore that extends into a porous formation, deposits from the drilling fluid will collect on wellbore wall and form a filter cake. The filter cake forms an important barrier between the formation fluids contained in the permeable formation at a certain pore pressure and the wellbore fluids that are circulating at a higher pressure. Thus, the filter cake provides a buffer that allows wellbore pressure to be maintained above pore pressure without significant losses of drilling fluid into the formation.
In order to maximize the rate of drilling, it is desirable to maintain the wellbore pressure at a level above, but relatively close to, the pore pressure. As wellbore pressure increases, drilling rate will decrease, and if the wellbore pressure is allowed to increase to the point it exceeds the formation fracture pressure (fracture pressure), a formation fracture can occur. Once the formation fractures, returns flowing in the annulus may exit the open wellbore thereby decreasing the fluid column in the well. If this fluid is not replaced, the wellbore pressure can drop and allow formation fluids to enter the wellbore, causing a kick and potentially a blowout. Therefore, the formation fracture pressure defines an upper limit for allowable wellbore pressure in an open wellbore. Typically, the formation immediately below the casing shoe has the lowest fracture pressure in the open wellbore, and therefore it is the fracture pressure at this depth that controls the maximum annulus pressure.
The fracture pressure is determined in part by the overburden acting at a particular depth of the formation. The overburden includes all of the rock and other material that overlays, and therefore must be supported by, a particular level of the formation. In an offshore well, the overburden includes not only the sediment of the earth but also the water above the mudline. The density of the earth, or sediment, provides an overburden gradient of approximately 1 psi per foot. The density of seawater provides an overburden gradient of approximately 0.45 psi/ft. The pore pressure at a given depth is determined in part by the hydrostatic pressure of the fluids above that depth. These fluids include fluids within the formation below the seafloor/mudline plus the seawater from the seafloor to the sea surface. A formation fluid gradient of 0.465 psi/ft is often considered normal. The typical seawater pressure gradient is about 0.45 psi/ft.
In surface and shallow water wells the differential in gradient between the seawater (or groundwater) and the earth often creates a pore pressure profile and fracture pressure profile that provide a sufficient range of pressure to allow the use of conventional drilling techniques. FIG. 1 shows a schematic representation of pore pressure PP and fracture pressure FG. The pressure developed in the wellbore is essentially determined by the hydrostatic pressure of the wellbore fluid, along with pressure variations due to fluid circulation and/or pipe movement. For any given open hole interval, the region of allowable pressure lies between the pore pressure profile, and the fracture pressure profile for that portion of the well between the deepest casing shoe and the bottom of the well.
Clean drilling fluid is circulated into the well through the drill string and then returns to the surface through the annulus between the wellbore wall and the drill string. In offshore drilling operations, a riser is used to contain the annulus fluid between the sea floor and the drilling rig located on the surface. The pressure developed in the annulus is of particular concern because it is the fluid in the annulus that acts directly on the uncased borehole.
The fluid flowing through the annulus, typically known as returns, includes the drilling fluid, cuttings from the well, and any formation fluids that may enter the wellbore. The drilling fluid typically has a fairly constant density and thus the hydrostatic pressure in the wellbore vs. depth can typically be approximated by a single gradient starting at the top of the fluid column. In offshore drilling situations, the top of the fluid column is generally the top of the riser at the surface platform.
The pressure profile of a given drilling fluid varies depending upon whether the drilling fluid is being circulated (dynamic) or not being circulated (static). These two pressure profiles are represented by the static pressure SP and dynamic pressure DP profiles on FIG. 1. In the dynamic case, there is a pressure loss as the returns flow up the annulus between the drill string and wellbore wall. This pressure loss adds to the pressure of the drilling fluid in the annulus. Thus, this additional pressure must be taken into consideration to ensure that drilling is maintained in an acceptable pressure range between the pore pressure gradient and fracture pressure gradient profile.
Because the dynamic pressure DP is higher than the static pressure SP, it is the dynamic pressure at the highest point in the uncased wellbore, i.e. the lowermost casing shoe, that is limited by the fracture pressure FG at depth D1. Correspondingly, the lower static pressure SP must be maintained above the pore pressure PP at the deepest point D2 in the open wellbore. Therefore, the range of allowable pressures for a certain length of uncased wellbore L1, as shown in FIG. 1, is limited by the dynamic pressure DP reaching fracture pressure FG at the casing shoe depth D1 and the static pressure SP reaching pore pressure PP at the bottom of the well D2.
Thus, in common drilling practice, the density of the drilling fluid will be chosen so that the dynamic pressure is as close as is reasonable to the fracture pressure at the casing shoe. This maximizes the depth that can then be drilled using that density fluid. Once the static pressure approaches pore pressure at the bottom of the well, another string of casing will be set and the same process repeated. Even when using conservative drilling techniques, the wellbore pressure may fall out of the acceptable range between pore pressure and fracture pressure and cause a kick. A kick may be recognized by drilling fluids flowing up through the annulus after pumping is stopped. A kick may also be recognized by a sudden increase of the fluid level in the drilling fluid storage tanks. After a kick has been detected, steps must be taken to control the kick.
There are two commonly used methods for controlling kicks, namely the driller""s method and the engineer""s method. In both methods the well is shut in and the wellbore pressure allowed to stabilize. The pressure will stabilize when the pressure at the bottom of the hole equalizes with formation pressure. The pressure indicated at the surface in the drill string and the casing annulus can be used to calculate the pressure at the bottom of the wellbore. With the well in the shut-in condition, the pressure at the bottom of the wellbore will be the formation pressure.
When using the driller""s method, once the wellbore pressure has stabilized, the pumps are restarted and drilling fluid is circulated through the well. The pressure within the casing is maintained so that no additional formation fluids flow into the well and fluid is circulated until any gas that has entered the wellbore has been removed. A higher density drilling fluid is then prepared and circulated through the well to bring the wellbore pressures back to within the desired pressure range. Thus, when killing a kick using the driller""s method, the fluid within the wellbore is fully circulated twice.
When using the engineer""s method, as the wellbore pressure stabilizes, the formation pressure is calculated. Based on the calculated formation pressure, a mixture of higher density drilling fluid is prepared and circulated through the well to kill the kick and circulate out any formation fluids in the wellbore. During this circulation, the annulus pressure is maintained until the heavy weight drilling fluid circulates completely through the well. Using the engineer""s method, the kick can be killed in a single circulation, as opposed to the two circulation driller""s method.
The key parameter for well control is determining the formation pressure and adjusting the wellbore pressure accordingly. If wellbore pressure is allowed to decrease below the pore pressure at a certain depth, formation fluids will enter the well. If wellbore pressure exceeds fracture pressure at a certain depth, the formation will fracture and wellbore fluids may enter the formation. Conventionally, downhole pressure is calculated using drill pipe and annulus pressures measured at the surface. To accurately measure these surface pressures, circulation is normally stopped, to allow the downhole pressure to stabilize and to eliminate any dynamic component of wellbore pressure, and the well is fully shut in. This, of course, uses valuable rig time and involves stopping drilling, which may cause other problems, such as a stuck drill string.
Some drilling operations seek to determine formation pressure using measurement while drilling (MWD) techniques. One deficiency of the prior art MWD methods is that many tools transmit pressure measurement data back to the surface on an intermittent basis. Many MWD tools incorporate several measurement tools, such as gamma ray sensors, neutron sensors, and densitometers, and typically only one measurement is transmitted back to the surface at a time. Thus, the interval between pressure data being reported may be as much as 2 minutes.
Transmitting the data back to the surface can be accomplished by one of several telemetry methods. One typical prior art telemetry method is mud pulse telemetry. A signal is transmitted by a series of pressure pulses through the drilling fluid. These small pressure variances are received and processed into useful information by equipment at the surface. Mud pulse telemetry does not work when fluids are not being circulated or are being circulated at a slow rate. Therefore, mud pulse telemetry and therefore standard MWD tools have very little utility when the well is shut in and fluid is not circulating.
Although MWD tools can not transmit data via mud pulse telemetry when the well is not circulating, many MWD tools can continue to take measurements and store the collected data in memory. The data can then be retrieved from memory at a later time when the entire drilling assembly is pulled out of the hole. In this manner, the operators can learn whether they have been swabbing the well, i.e. pulling fluids into the borehole, or surging the well, i.e. increasing the wellbore pressure, as the drill string moves through the wellbore.
Another telemetry method of sending data to the surface is electromagnetic telemetry. A low frequency radio wave is transmitted through the formation to a receiver at the surface. Electromagnetic telemetry is depth limited, and the signal attenuates quickly in water. Therefore, with wells being drilled in deep water, the signal will propagate fairly well through the earth but it will not propagate through the deep water. Thus, a subsea receiver would have to be installed at the mud line, which may not be practical.
Thus, there remains a need in the art for methods and apparatus for determining and adjusting wellbore pressure based on real-time pressure data received from the bottom of a well. Therefore, the embodiments of the present invention are directed to methods and apparatus for using real-time pressure data to automate pressure control procedures that seek to overcome the limitations of the prior art.
Accordingly, there are provided herein methods and apparatus for monitoring and controlling the pressure in a wellbore. The preferred embodiments of the present invention are characterized by a drilling system utilizing real-time bottom hole pressure measurements and a control system adapted to automatically control parameters such as drilling fluid weight, pumping rate, and choke actuation. In the preferred embodiments, the control system receives input from the bottom hole pressure sensor as well as pressure sensors, mud volume sensors, and flowmeters located at the surface. The control system then adjusts one or more of the drilling fluid density, pumping rate, or choke actuation to detect, shut-in, and circulate out wellbore influxes.
One preferred embodiment includes a method for detecting and controlling an influx of formation fluids into the wellbore when the drill bit is at the bottom of the hole. Once a kick is detected, either by downhole pressure sensing or by mass flow rate balancing, the well can be shut and the formation pressure measured by the downhole pressure sensor. The downhole pressure measurements may be made once circulation has stopped or while circulation continues. Once formation pressure has been established, the control system adjusts one or more of drilling fluid density, pumping rate, or choke actuation to circulate out wellbore influxes.
Thus, the present invention comprises a combination of features and advantages that enable it to use real-time downhole pressure data to substantially improve management of kicks and other wellbore pressure abnormalities. These and various other characteristics and advantages of the present invention will be readily apparent to those skilled in the art upon reading the following detailed description of the preferred embodiments of the invention and by referring to the accompanying drawings.